1. Field of the Invention
The present invention relates to flat display screen, and more particularly to so-called cathodoluminescent screens, the anode of which carries luminescent elements, separated from one another by insulating areas, and likely to be energized by electron bombardment from microtips.
2. Discussion of the Related Art
The accompanying drawing shows an example of a flat microtip color screen of the type to which the present invention relates.
Such a microtip screen is essentially comprised of a cathode 1 having microtips 2 and of a grid 3 provided with holes 4 corresponding to the locations of microtips 2. Cathode 1 is placed facing a cathodoluminescent anode 5, a glass substrate 6 of which constitutes the screen surface.
The operating principle and a specific embodiment of a microtip screen are described, in particular, in U.S. Pat. No. 4,940,916 to Borel et al.
Cathode 1 is organized in columns and is comprised, on a glass substrate 10, of cathode conductors organized in meshes from a conducting layer. Microtips 2 are implemented on a resistive layer 11 deposited on the cathode conductors and are placed inside the meshes defined by the cathode conductors. The drawing partially shows the inside of a mesh and the cathode conductors are not shown therein. Cathode 1 is associated with grid 3 which is organized in lines. The intersection of a line of grid 3 and of a column of cathode 1 defines a pixel.
The device uses the electric field which is created between cathode 1 and grid 3 to extract electrons from microtips 2. These electrons then are attracted by phosphor elements 7 of anode 5 if the latter are properly biased. In the case of a color screen, anode 5 is provided with alternating phosphor bands 7r, 7g, 7b, each corresponding to a color (Red, Green, Blue). The bands are parallel to the columns of the cathode and are separated from one another by an insulator 8, generally silicon oxide (SiO.sub.2). The phosphors 7 are deposited on electrodes 9, comprised of corresponding bands of a transparent conducting layer such as indium tin oxide (ITO). The sets of red, green, and blue bands are alternately biased with respect to cathode 1, so that electrons extracted from the microtips 2 of a pixel of the cathode/grid are alternately directed towards phosphors 7 facing each of the colors.
The selection control of phosphor 7 (phosphor 7g in the drawing) which is to be bombarded by the electrons from the microtips of cathode 1 imposes to control, selectively, the bias of the phosphors 7 of anode 5, color per color.
Generally, the rows of grid 3 are sequentially biased to a potential of approximately 80 volts, whereas the phosphor bands (for example, 7g) to be energized are biased under a voltage of approximately 400 volts via the ITO band on which the phosphors are deposited. The ITO bands, carrying the other phosphor bands (for example 7r and 7b), are at a low or zero potential. The columns of cathode 1 are brought to respective potentials included between a maximum emission potential and a zero emission potential (for example, respectively 0 and 30 volts). The brightness of a color component of each of the pixels in a line thus is set.
The selection of the values of the bias potentials is linked with the characteristics of phosphors 7 and of microtips 2. Conventionally, below a potential difference of 50 volts between the cathode and the grid, there is no electron emission, and the maximum emission used corresponds to a potential difference of 80 volts.
A disadvantage of conventional screens is that the microtips progressively lose their emitting power. This phenomenon can be acknowledged by measuring the current through the cathode conductors. As a result, the screen brightness progressively decreases, which is prejudicial to the lifetime of conventional screens.